The present invention is directed to a process for forming bi-axially highly textured extended objects (such as tapes and wires) and bulk objects (including single-crystals), by pre-aligning materials using a variety of approaches and then applying a strong magnetic field (H.gtoreq.10.sup.4 Oe) and high (near melting point) temperatures to materials having anisotropy in the paramagnetic susceptibility.
Standard conductors obey Ohm's law. As charges move through a conductor they produce an electric field that creates an electric current. The current density (J) of a conductor having a cross-sectional area A is defined as follows: EQU J=I/A
where I is the current running through the conductor. If a constant potential difference is maintained across a conductor, the current (I) remains constant. If the current density in a conductor is proportional to the electric field within the conductor, the conductor is said to obey Ohm's law: EQU J=.sigma.E
where .sigma. is the constant of proportionality and E is the electric field strength.
The resistance of a conductor may be calculated according to the following expression: EQU R=V/I
where V is the potential difference, and the resistivity of a material is found according to the following expression: EQU .rho.=l/.sigma.
In the early part of this century it was discovered that resistance dropped to zero in some materials, such as mercury (Hg), at low temperatures. Materials having an electrical resistance of zero (R=0) are called superconductors. Such materials do not obey Ohm's law.
Theories such as the BCS Theory of Superconductivity and the Cooper Electron Pairing Theory attempt to explain the lack of resistance in superconducting materials below T.sub.c (temperature at which resistance equals zero). While the BCS theory has not been entirely successful with new high T.sub.c ceramics the Cooper Electron Pairing Theory is commonly considered valid.
Recent developments in superconductor technology such as Paul Chu's synthesis of a material superconducting at 95.degree.-96.degree. K. (YBa.sub.2 Cu.sub.3 O.sub.x, where x.apprxeq.7) and then the recent discoveries of Bi- and Tl-based superconducting ceramic materials (for instance, Tl(Bi)Ba.sub.2 Ca.sub.n-1 Cu.sub.n O.sub.2n+3 where n in an integer selected from the set 1, 2, 3, and 4) with a highest T.sub.c of 125.degree. K. have lead to very promising directions.
Now that economical T.sub.c 's have been achieved, new problems must be confronted and solved. The most crucial problem is critical current density J.sub.c (maximum current density at a given temperature and magnetic field above which a material ceases to be superconducting). This is true since low current densities (J.sub.c) require larger HTSC (high termperature superconducting ceramics) cross-sections. Naturally, this results in a cost increase. The problem can be addressed by making better quality materials that allow for a significantly larger J.sub.c. The objective of the present invention is to make HTSC materials with high J.sub.c 's.
For many technological applications J.sub.c should have a magnitude of about 10.sup.5 to 10.sup.6 A/cm.sup.2 at 77 K. In single-crystals RE-Ba.sub.2 Cu.sub.3 O.sub.x (where RE is a rare earth) a J.sub.c of .apprxeq.3.times.10.sup.6 A/cm.sup.2 and higher has been obtained. See, S. Jin, et al. 51 Applied Physics Letters 203 et seq. (1987).
However, J.sub.c is strongly anisotropic and is sufficient only for certain directions (for current flow in the Cu-O basal plane). In polycrystalline untextured bulk and elongated samples, where grains are randomly oriented, superconducting current flows along "good" directions in some grains and along "bad" directions in other grains, which results in an unacceptably low J.sub.c (.apprxeq.10.sup.2 A/cm.sup.2). Additionally, a mismatch in the a-b planes has an adverse effect on J.sub.c. This adverse effect manifests as so called weak links between adjacent grains.
One logical approach to enhancing J.sub.c is to prepare grain oriented polycrystalline ceramics, or to turn or regrow grains in such a way that current flows along "good" directions only. It is furthermore crucial to liquidate the mismatch in a-b axes registry. When both these goals are achieved, it becomes possible to attain very high J.sub.c 's in textured compacts approaching the ones achieved on the single-crystal samples.
Textured material is a material in which the vast majority of the grains within the material have the same crystallographic orientation with respect to some reference direction. The highest probability texture direction is called the "preferred orientation."
A texture is often specified with respect to the external directions of the material under consideration, for example, to the plane and edges of a tetragonally shaped bulk sample, or to the axis of a wire. Texture may be uniaxial or planar, such as "c-axis texture," wherein the c-axes of the majority of grains are approximately parallel to one another. Texture may also be bi-axial or three-dimensional, when not only the c-axis of the majority of grains is aligned but one additional axis, located within the a-b plane, for the majority of grains is also aligned. In the latter case the grained polycrystalline material most closely matches the lattice characteristics associated with single crystals.
Very often textured materials have superior conducting, mechanical, electromagnetic, wave alternating and transducing properties, etc., and the demand for such material is increasing. For example, in the area of high temperature superconducting ceramics (HTSC) highly grain-oriented materials are desirable because they exhibit high critical current densities, as well as high critical magnetic fields favored in single crystals and textured samples.
One method of producing texture is to influence the grain growth process. As grain growth occurs in a material, some grains grow at the expense of their neighbors. If a means can be found to enhance the growth of grains selected on the basis of their crystallographic orientations, then highly textured material in which the vast majority of the grains are crystallographically oriented may be obtained.
A common method of producing such a selection mechanism is to utilize a temperature gradient during grain growth for materials with a large anisotropy in crystal growth directions (used in, for example, melt-textured growth technique). Another approach is to utilize mechanical pressure. For example, rolling thin sheets of certain metals causes preferential grain alignment.
In the present invention grains in the first step are uniaxially prealigned by using a magnetic field, compression or other means. In a second step a difference in the magnetic component of energy is provided between grains favorably and unfavorably oriented with respect to the direction of an applied magnetic field. This difference in energy is due to two factors, (1) anisotropy in the paramagnetic/diamagnetic susceptibility (the difference in the grain magnetic susceptibilities in the directions parallel and not parallel to the magnetic field); and (2) magnitude of the magnetic field itself (the energy term when the atomic magnetic moments are not saturated is proportional to the square of the magnetic field).
In another version of the process, the material is heated to a temperature exceeding its melting point, then it is cooled in the presence of a temperature gradient while a strong magnetic field is simultaneously applied. Directions of the temperature gradient and magnetic field select favored crystallographic orientations during melt (or partial melt) solidification. Additionally, the strong magnetic field may rotationally realign grains and grain nucleous in the melt in a preferred direction. The stronger the magnetic field, the faster may be the cooling rate.
This implies that in order to maximize the magnetic energy term it is necessary to use the maximum achievable magnetic field. Magnetic fields of 10.sup.4 to 2.times.10.sup.5 Oe are currently producible with commercially available equipment. Magnetic fields of the strength above 3.times.10.sup.5 Oe are available in a few of the best magnetic facilities in the world.
When a material with an anisotropic magnetic susceptibility is placed in a magnetic field, the energy of grains favorably oriented with respect to the field direction is lower than that of other orientations.
When a material is heated to a near melting point temperature and grain growth occurs, the larger size grains, due to a surface energy term (surface energy density is smaller for grains having smaller curvature (or larger size)), expand at the expense of the smaller grains. This process does not generally result in a preferred orientation. However, if a material is placed in a sufficiently strong magnetic field, so that the magnetic term dominates the surface energy term, the grains with favorable crystallographic orientation will grow at the expense of adjacent unfavorably oriented grains without regard to their size. Additionally, if a sufficient amount of liquid phase is present, grains can be rotationally realigned in the favorable direction. This process results in textured samples.